Image sensors with organic photodiodes and methods for forming the same

ABSTRACT

Embodiments of forming an image sensor with organic photodiodes are provided. Trenches are formed in the organic photodiodes to increase the PN-junction interfacial area, which improves the quantum efficiency (QE) of the photodiodes. The organic P-type material is applied in liquid form to fill the trenches. A mixture of P-type materials with different work function values and thickness can be used to meet the desired work function value for the photodiodes.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. application Ser. No.16/017,707, filed on Jun. 25, 2018, which is a continuation of U.S.application Ser. No. 15/242,220, filed on Aug. 19, 2016, no U.S. Pat.No. 10,008,546, which is a continuation of U.S. application Ser. No.14/012,713, filed on Aug. 28, 2013, now U.S. Pat. No. 9,425,240, whichapplications are hereby incorporated herein by reference.

The present application is related to U.S. application Ser. No.14/012,789, entitled “Organic Photodiode with Dual Electron-BlockingLayers,” and U.S. application Ser. No. 14/012,692, entitled “OrganicPhotosensitive Device with an Electron-Blocking and Hole-TransportLayer.” Both above-mentioned applications are incorporated herein byreference in their entireties.

BACKGROUND

Image sensor chips, which include front side illumination (FSI) imagesensor chips and Backside Illumination (BSI) image sensor chips, arewidely used in applications such as cameras. In the formation of imagesensor chips, image sensors (such as photo diodes) and logic circuitsare formed on a silicon substrate (or a wafer), followed by theformation of an interconnect structure on a front side of the wafer. Inthe front side image sensor chips, color filters and micro-lenses areformed over the interconnector structure. In the formation of the BSIimage sensor chips, after the formation of the interconnect structure,the wafer is thinned, and backside structures such as color filters andmicro-lenses are formed on the backside of the silicon substrate. Whenthe image sensor chips are used, light is projected on the imagesensors, in which the light is converted into electrical signals.

The image sensors in the image sensor chips generate electrical signalsin response to the stimulation by photons. Quantum efficiency (QE) of aphotosensitive device measures a percentage of photons hitting adevice's photoreactive surface that produce charge carriers.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a top view of an image sensor, in accordance with someembodiments.

FIG. 2 is a cross-sectional view of the image sensor of FIG. 1, inaccordance with some embodiments.

FIG. 3 is a cross-sectional view of a photodiode, in accordance withsome embodiments.

FIG. 4A is a cross-sectional view of an N-type layer formed over anelectrode layer, in accordance with some embodiments.

FIGS. 4B and 4C are top views of a patterned N-type layer of FIG. 4A, inaccordance with some embodiments.

FIG. 5 is a cross-sectional view of a photodiode, in accordance withsome embodiments.

FIG. 6A is a cross-sectional view of an electrode layer of a photodiodewith trenches, in accordance with some embodiments.

FIG. 6B is a cross-sectional view of an N-type layer and an electrontransport layer over the electrode layer of FIG. 6A, in accordance withsome embodiments.

FIGS. 6C and 6D are top views of a patterned electrode layer of FIG. 6A,in accordance with some embodiments.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof various embodiments. Specific examples of components and arrangementsare described below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Moreover, the formation of a first feature over or on a second featurein the description that follows may include embodiments in which thefirst and second features are formed in direct contact, and may alsoinclude embodiments in which additional features may be formedinterposing the first and second features, such that the first andsecond features may not be in direct contact.

Referring to FIG. 1, an image sensor 50 provides a grid of front sideilluminated (or front-illuminated) pixels 100, in accordance with someembodiments. In at least the present embodiment, the pixels 100 arephotosensitive diodes or photodiodes, for recording an intensity orbrightness of light on the diode. The pixels 100 include resettransistors, source follower transistors, and transfer transistors,etc., in some embodiments. In some embodiments, the image sensor 50includes a charge-coupled device (CCD), a complimentary metal oxidesemiconductor (CMOS) image sensor (CIS), an active-pixel sensor (ACP),or a passive-pixel sensor. In some embodiments, organic photodetectors(sensors of light) made with organic semiconductor materials(non-silicon based material) are used due to low manufacturing cost.Such organic semiconductor materials are able to be easily integratedwith CMOS process technology. Additional devices and circuitry aretypically provided adjacent to the grid of pixels 100 for providing anoperation environment for the pixels and for supporting externalcommunications with the pixels. In some embodiments, the additionaldevices and circuitry are made by CMOS process technology. As a result,image sensors using organic photodetectors with CMOS process technologyare called hybrid CMOS image sensors.

As mentioned above, organic semiconductor materials are attractive dueto low manufacturing cost. However, the existing image sensors usingorganic semiconductor materials either have low quantum efficiency (QE)or high dark current. As a result, there is a need for image sensorsusing organic semiconductor materials with high QE and low dark current.

FIG. 2 is a cross-sectional view of image sensor 50, in accordance withsome embodiments. Sensor 50 includes a silicon substrate 110, inaccordance with some embodiments. In some embodiments, the substrate nocomprises an elementary semiconductor such as silicon, germanium, ordiamond. In some embodiments, the substrate no comprises a compoundsemiconductor such as silicon carbide, gallium arsenic, indium arsenide,or indium phosphide. Also, semiconductor arrangements such assilicon-on-insulator and/or an epitaxial layer are provided, in someembodiments. In some embodiments, the substrate no comprises an alloysemiconductor such as silicon germanium, silicon germanium carbide,gallium arsenic phosphide, or gallium indium phosphide. In at least thepresent embodiment, the substrate 110 comprises P-type silicon. Alldoping is able to be implemented using a process such as ionimplantation or diffusion in various steps.

The image sensor 50 includes a plurality of pixels 100, such as 100R,100G, and 100B, formed on the front surface of the semiconductorsubstrate 110. For the sake of example, the pixels shown in FIG. 2 arefurther labeled 100R, 100G, and 100B to correspond with example lightwavelengths of red, green, and blue, respectively.

The image sensor 50 further includes an interconnect structure 126,which includes additional layers, such as first and second metal layers120, 122, contacts/vias 119, 121, 123 and inter-level dielectric (ILD)124. The metal layers and contacts/vias are formed by single or dualdamascene process, in some embodiments. If the metal layers andcontacts/vias are formed by dual damascene process, a metal layer, suchas layer 122, and corresponding vias, such as vias 121, are formed by adeposition process, such as a plating process. Contacts 119 connectfirst metal layer 120 with device structures 115. FIG. 2 indicates thatcontacts 119 connect to source/drain regions 114 of device structures115. However, contacts 119 may also connect to gate structures 113. Vias123 connect interconnect of device structures 115 with a pixel electrodelayer 170, which are part of pixels 100. Device structures 115 indifferent pixels 100 are separated by isolation structures 112, such asshallow trench isolation (STI) structures.

The ILD 124 includes multiple layers of dielectric films, which includeone or more low-k materials. Low-k materials have a lower dielectricconstant than a dielectric constant of silicon dioxide. In someembodiments, ILD 124 includes carbon-doped silicon oxide, fluorine-dopedsilicon oxide, silicon oxide, silicon nitride, organic low-k material,or combinations thereof. In some embodiments, the conductive material(s)of metal layers, such as 120 and 122, and contacts/vias 119, 121 and 123include aluminum, aluminum alloy, copper, copper alloy, tungsten,titanium, titanium nitride, tantalum, tantalum nitride, metal silicide,tungsten or combinations thereof.

Additional circuitry also exists to provide appropriate functionality tohandle a type of pixels 100 being used and a type of light being sensed.One of ordinary skill would understand that the wavelengths red, green,and blue are provided for the sake of example only, and that the pixels100 include photodiodes 135 as photosensors. Photodiodes 135 usesorganic semiconductor materials, in some embodiments. Details ofmaterial layers used in forming organic photodiodes 135 are describedbelow.

In some embodiments, there are light-blocking structures 125 betweenpixels 100. Light-blocking structures 125 block the transmission oflight from neighboring color filters 160 and reduce cross-talk betweenpixels 100.

The image sensor 50 is designed to receive light 150 directed towards afront surface of the semiconductor substrate no during operation,eliminating obstructions to the optical paths by other objects such asgate features and metal lines, and maximizing the exposure of alight-sensing region to an illuminated light. The illuminated light 150is not limited to visual light beam, and is able to include infrared(IR), ultraviolet (UV), or other proper radiation beam.

The image sensor 50 further includes a color filter layer 160 over apassivation layer 130. The passivation layer 130 protects the pixels 100from being damaged during formation of color filters, such as 160R,160G, and 160B, and micro-lenses 140. In some embodiments, passivationlayer 130 is made of a dielectric material, such as silicon oxide,silicon nitride, silicon oxynitride, polymers, such as polyimide,polybenzoxazole (PBO), or benzocyclobutene (BCB), or combinationsthereof. The color filter layer 160 is able to support several differentcolor filters (e.g., red, green, and blue), and are positioned such thatthe incident light is directed thereon and there through. In at leastone embodiment, such color filters 160R, 160G, and 160B comprise apolymeric material (e.g., negative photoresist based on an acrylicpolymer) or resin. In some embodiments, the color filter layer 160includes a negative photoresist based on an acrylic polymer includingcolor pigments. In some embodiments, color filters 160R, 160G, and 160Bcorrespond to pixels 100R, 100G, and 100B, respectively.

The image sensor 50 includes a number of micro-lens 140 formed overcolor filters 160R, 160G, and 160B. Micro-lens 140 focus illuminatedlight 150 toward pixels 100R, 100G, and 100B.

FIG. 3 is a cross-sectional view of a photodiode 135, in accordance withsome embodiments. Each photodiode 135 has two electrodes. Photodiode 135includes a pixel electrode layer 170, which is made of a conductivematerial and is formed over interconnect structures 126, as shown inFIG. 2. Pixel electrode layer 170 forms a cathode of the photodiode ofpixel 100. A work function of the electrode layer 170 is less than about4.5 eV, in some embodiments. In some embodiments, pixel electrode layer170 includes Ag, Al, Ca, Mg, or other applicable conductive materials.In some embodiments, a thickness of electrode layer 170 is in a rangefrom about 50 nm to about 500 nm. The work function of pixel electrodelayer 170 can be tuned, by choosing a material and by varying thethickness. In some embodiments, the pixel electrode layer 170 isdeposited by physical vapor deposition (PVD). However, other depositionmethod, such as chemical vapor deposition (CVD) or atomic layerdeposition (ALD), are also possible.

An electron transport layer 171 is formed over pixel electrode layer170, in some embodiments. The electron transport layer 171 provides awork function that assists transport of electron and blocks transport ofholes. Therefore, electron transport layer 171 is also called a holeblocking layer. In some embodiments, the work function of the electrontransport layer 171 has a range of work function values. A highest workfunction value is called highest occupied molecular orbital (HOMO) and alowest work function value is called lowest occupied molecular orbital(LUMO). In some embodiments, the LUMO of electron transport layer 171 isin a range from about 2.8 eV to about 4.5 eV and the HOMO of electrontransport layer 171 is in a range from about 6.1 eV to about 7.8 eV. Insome embodiments, electron transport layer 171 is made of LiF, TiO₂,ZnO, Ta₂O₅, ZrO₂, or other applicable conductive materials. The workfunction of electron transport layer 171 is able to be tuned, bychoosing the material and by varying the thickness. Electron transportlayer 171 is omitted in some embodiments. In some embodiments, theelectron transport layer 171 is deposited by physical vapor deposition(PVD). However, other deposition method, such as chemical vapordeposition (CVD) or atomic layer deposition (ALD), are possible.

Afterwards, an N-type layer 172 is deposited over electron transportlayer 171, or electrode layer 170, if electron transport layer 171 isnot present. The N-type layer 172 and a subsequent P-type layer, whichis to be described below, form P-N junction photodiodes. In someembodiments, the N-type layer 172 is made of a metal oxide, such as ZnO,TiO₂, or other suitable metal oxide materials. In some embodiments, theLUMO of N-type layer 172 is in a range from about 3.7 eV to about 4.5 eVand the HOMO of N-type layer 172 is in a range from about 6.7 eV toabout 7.8 eV.

A thickness TT of the N-type layer 172 is in a range from about 50 nm toabout 300 nm, in accordance with some embodiments. Similarly, the workfunction of N-type layer 172 can be tuned, by choosing the material andby varying the thickness. In some embodiments, the N-type layer 172 isdeposited by physical vapor deposition (PVD). However, other depositionmethod, such as chemical vapor deposition (CVD) or atomic layerdeposition (ALD), are also possible.

The N-type layer 172 is then patterned to form trenches 177 to increasea surface area of the N-type layer 172, which increases the interfacialarea of P-N junctions of the photodiode 135. As mentioned above, quantumefficiency (QE) of a photosensitive device measures the percentage ofphotons hitting a device's photoreactive surface that produces chargecarriers. Photodiodes with large PN junction interfacial area enable tomore effectively convert photons hitting the diodes into chargecarriers. As a result, QE is increased by the inclusion of trenches 177.In some embodiments, the QE is improved in a range from about 20% toabout 200%, as compared to photodiodes with flat surfaces, which haslower PN-junction interfacial area.

FIG. 4A is a cross-sectional view of a patterned N-type layer 172 formedover pixel electrode layer 170, in accordance with some embodiments. Asmentioned above, trenches 177 are formed in layer 172. Trenches 177 havewidths WT in a range from about 10 nm to about 200 nm, in someembodiments. The widths WT of different trenches 177 are equal in someembodiments. However, the widths WT of at least one trench 177 isdifferent from at least one other trench 177, in some embodiments. Thewidth WN of the N-type layer 172 between two neighboring trenches 177 isin a range from about 10 nm to about 200 nm in some embodiments. Thedepths DT of trenches 177 are in a range from about 10 nm to about 290nm. The depths DT of different trenches 177 are equal in someembodiments. However, the depth DT of at least one trench 177 isdifferent from at least one other trench 177, in some embodiments. Atotal thickness TT of the N-type layer 172 is in a range from about 50nm to about 300 nm, in some embodiments. A remaining thickness TR ofN-type layer 172 below bottoms of trenches 177 to the surface of layer170 is in a range from about 10 nm to about 290 nm, in some embodiments.The trenches 177 are formed by first patterning the surface of layer 172with a photoresist layer (not shown) and then using an etch process toremove N-type layer 172 not covered by the photoresist layer.

FIGS. 4B and 4C are top views of patterned layer 172 of FIG. 4A, inaccordance with some embodiments. FIG. 4B includes trenches 177 beinglong and continuous trenches separating long and solid (and continuous)strips of N-type layer 172. FIG. 4C includes that long and continuoustrenches 177 run in both X and Y directions. Trenches 177 separatesislands of N-type layer 172. FIGS. 4B and 4C are two examples. Otherconfigurations and top views of N-type layer 172 are also possible. Theexistence of the trenches 177 increases the surface area of N-type layer172 to form P-N junction with subsequent P-type layer. The large P-Njunction interface enables absorbing more light per pixel to produceelectrical signals with higher intensity.

After N-type layer 172 is patterned, an organic P-type layer 173 isdeposited over the patterned N-type layer 172. The organic P-type layer173 is made of an organic semiconductor material. In some embodiments,the organic P-type layer 173 is made of a conjugated polymer. Theexample, applicable the conjugated polymers include thiophene-basedconjugated polymer, such as poly(3-hyxylthiophene) (P3HT),benzodithiophene-based conjugated polymer,thieno[3,4-c]pyrrole-4,6-dione (TPD)-based conjugated polymer,diketo-pyrrole-pyrrole (DPP)-based conjugated polymer, bithiazole(BTz)-based conjugated polymers, benzothiadiazole (BT)-based conjugatedpolymer, thieno[3,2-b]thiophene (TT)-based conjugated polymer, or acombination thereof. In some embodiments, the organic P-type layer 173is formed by mixing the selected conjugated polymer(s), such as P3HT andan aromatic solvent(s), such as toluene and/or 1,2-dichlorobenzene, andthen stirring the mixture at an elevated temperature. In someembodiments, the elevated temperature is at about 600° C. The mixture ofconjugated polymer(s) and solvent (in liquid form) is then applied, suchas by spraying with a nozzle, on the surface of N-type layer 172. Thesolvent(s) is then driven out by evaporation. Since the mixture of theorganic P-type materials, such as P3HT, and solvent(s) are in fluid formwhen it is applied on the surface of N-type layer 172, the mixture fillstrenches 177 and leaves a substantially flat surface 178.

In some embodiments, a LUMO of P-type layer 173 is in a range from about2.8 eV to about 3.6 eV and a HOMO of P-type layer 173 is in a range fromabout 4.5 eV to about 5.6 eV. The lower values, such as 2.8 eV and 4.0eV are close to the work function of the pixel electrode layer 170(cathode). The higher values, such as 4.5 eV and 5.6 eV, are close tothe work function of an anode.

A thickness DP of the organic P-type layer 173 over the top surface ofN-type layer 172 is in a range from about 20 nm to about 300 nm, asshown in FIG. 3 in accordance with some embodiments. The thickness DPTof the organic P-type layer 173 over the bottom surface of N-type layer172 is in a range from about 30 nm to about 500 nm. The thickness of theP-type layer is adjusted with different types of organic P-typematerials used. The P-type layer 173 and N-type layer 172 forms a P-Njunction 180, which is an interface between a P-type and an N-typematerials, of a photodiode.

Referring back to FIG. 3, after the organic P-type layer 173 isdeposited, a hole transport (or electron blocking) layer 174 isdeposited over the organic P-type layer 173, in some embodiments. Thehole transport layer 174 has a work function that assists the transportof holes and blocks the transport of electrons. In addition, the holetransport layer 174 is transparent, which allows light (150 as shown inFIG. 2) to pass through and reach the P-N junction 180.

In some embodiments, the hole transport layer 174 is made of metaloxide, such as MoO₃, WO₃, NiO, CuO, V₂O₅, etc. Alternatively, the holetransport layer 174 is made of polymers, such as a combination of(poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Awork function of the hole transport layer 174 is between the workfunction of the organic P-type layer 174 and the anode (to be describedbelow).

In some embodiments, a LUMO of hole transport layer 174 is in a rangefrom about 1.5 eV to about 3.0 eV and a HOMO of hole transport layer 174is in a range from about 4.8 eV to about 5.6 eV. For example, the workfunction of PEDOT:PSS is about 5.0 eV. If a metal oxide, such as MoO₃,WO₃, NiO, CuO, V₂O₅, etc., is used for the hole transport layer 174, thehole transport layer is able to be deposited by physical vapordeposition (PVD), chemical vapor deposition (CVD), atomic layerdeposition (ALD), or other applicable process. In some embodiments, athickness of hole transport layer 174 is in a range from about 1 nm □mto about 20 nm. As mentioned above, the work function of layer 174 canbe tuned, by choosing the material and by varying the thickness.

After the hole transport layer 174 is deposited, a transparent electrodelayer 175 is deposited over hole transport layer 174. The transparentelectrode layer 175 is transparent to allow light 150 to shine throughand is conductive to function as an electrode. Examples of suitablematerials for transparent electrode layer 175 include, but are notlimited to, indium tin oxide (ITO), indium zinc oxide (IZO), indiumgallium zinc oxide (IGZO), etc. In some embodiments, transparentelectrode layer 175 is deposited by physical vapor deposition (PVD).However, other deposition processes, such as chemical vapor deposition(CVD) or atomic layer deposition (ALD), or other applicable processes,are possible.

In some embodiments, a thickness of transparent electrode layer 175 isin a range from about 50 nm to about 300 nm. In some embodiments, a workfunction of transparent electrode layer 175 is in a range from about 4.5eV to about 5.5 eV. The work function of layer 175 can be tuned, bychoosing the material and by varying the thickness.

As mentioned above, there are light-blocking structures 125 betweenphotodiodes 135. After transparent electrode layer 175 is deposited, apatterning process is performed to remove layers, such as layers170-175, between photodiodes 135. The patterning process includesforming a photoresist layer (not shown), with openings that correlate toisolation structures 125, and etching to remove a portion of thematerial layers of pixels wo under the openings. Afterwards, etching iscompleted, the remaining photoresist layer is removed. A non-transparentdielectric material is then deposited to fill the openings and to formlight-blocking structures 125.

FIG. 3 includes a pixel 100 with large P-N junction interface 180 due tothe large surface area of the N-type layer 172. There are othermechanisms to produce large areas of P-N junction interface. FIG. 5includes a cross-sectional view of a photodiode 135′, in accordance withsome embodiments. Photodiode 135′ is similar to photodiode 135 describedabove. However, the large P-N junction interface 180′ of photodiode 135′is created by forming trenches in the pixel electrode layer 170′,instead of N-type layer 172 for photodiode 135, as shown in FIG. 5. FIG.5 shows that photodiode 135′ includes various layers, 170′-175′ similarto layers, 170-175, of photodiode 135 of FIG. 3. However, the largesurface of N-type layer 172′ is created by forming trenches in pixelelectrode layer 170′.

The material and function for pixel electrode layer 170′ is similar topixel electrode layer 170 described above. As described above, pixelelectrode layer 170′ is made of a conductive material and is formed overinterconnect structures 126. Pixel electrode layer 170′ forms thecathode of the photodiode of pixel 100. In some embodiments, a totalthickness TE of electrode layer 170′ is in a range from about 100 nm toabout 500 nm, in some embodiments. The thickness of layer 170′ in theembodiment shown in FIG. 5 is larger than layer 170 in the embodimentshown in FIGS. 4A-4C. The material suitable for layer 170′ has beendescribed above. Layer 170′ is patterned to form trenches 177′, as shownin FIG. 6A. Widths WT of trenches 177′ are in a range from about 50 nmto about 450 nm, in some embodiments. The widths WT of trenches 177′ areequal in some embodiments. However, the widths WT′ of trenches 177′ varyamong different trenches 177′, in some embodiments. Depths DT′ oftrenches 177′ are in a range from about 50 nm to about 450 nm. Thedepths DT′ of trenches 177′ are equal in some embodiments. However, thedepths DT′ of trenches 177′ vary among different trenches 177′, in someembodiments.

A width WE of the pixel electrode layer 170′ between two neighboringtrenches 177′ is in a range from about 20 nm to about 1 μm, in someembodiments. After trenches 177′ are formed in electrode layer 170′, anelectron transport layer 171′ is formed over layer 170, in someembodiments. The electron transport layer 171′ is similar to layer 171described above in terms of its function and its material. In someembodiments, a thickness of electron transport layer 171′ is in a rangefrom about 0.1 nm to about 20 nm, in some embodiments.

Electron transport layer 171′ covers the surface pixel electrode layer170′, including the surfaces of trenches 177′, as shown in FIG. 6B inaccordance with some embodiments. In some embodiments, electrontransport layer 171′ may be deposited by physical vapor deposition(PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD),or other suitable methods. A deposition method that forms a conformallayer would be desirable. Electron transport layer 171′ is omitted insome embodiments.

Afterwards, an N-type layer 172′ is deposited over electron transportelectron transport layer 171′, or pixel electrode layer 170′, ifelectron transport layer 171′ does not exist. The N-type layer 172′ issimilar to N-type layer 172 described above. The N-type layer 172′ andthe subsequently deposited P-type layer form P-N junction photodiodes. Athickness of the N-type layer 172′ is in a range from about 10 nm toabout 100 nm, in accordance with some embodiments.

N-type layer 172′ covers the surface electron transport layer 171′, asshown in FIG. 6B in accordance with some embodiments. N-type layer 172′is deposited by physical vapor deposition (PVD), chemical vapordeposition (CVD), atomic layer deposition (ALD), or other suitablemethods. A deposition method that forms a conformal layer would bedesirable. Due to the formation of trenches 177′ in the pixel electrodelayer 170′, the surface area of n-type layer 172′ is much larger than aphotodiode structure with flat layers and without forming trenches 177′.

After N-type layer 172′ is deposited, an organic P-type layer 173′ isdeposited over the patterned N-type layer 172′. Organic P-type layer173′ is similar to organic P-type layer 173 described above. The P-typelayer 173′ fills the trenches between N-type layer 172′ and leaves asubstantially flat surface 178′, as shown in FIG. 6A in accordance withsome embodiments.

A thickness DP′ of the organic P-type layer 173′ over a top surface ofN-type layer 172′ is in a range from about 20 nm to about 300 nm. Athickness DPT of the organic P-type layer 173′ over a bottom surface ofN-type layer 172′ is in a range from about 30 nm to about 500 nm. Thethickness of the P-type layer is adjusted with different types oforganic P-type materials used. The P-type layer 173′ and N-type layer172′ forms a P-N junction 180′, which is an interface between a P-typeand an N-type materials, of a photodiode.

After the organic P-type layer 173′ is formed, a hole transport (orelectron blocking) layer 174′ is deposited over the organic P-type layer173′. Hole transport layer 174′ is similar to hole transport layer 174described above. A transparent electrode layer 175′ is then depositedover hole transport layer 174′. The transparent electrode layer 175′ issimilar to transparent electrode layer 175 described above.

FIGS. 6C and 6D are top views of patterned pixel electrode layer 170′ ofFIG. 6A, in accordance with some embodiments. FIG. 6C includes trenches177′ being long and continuous trenches separating long and solid (andcontinuous) strips of layer 170′. FIG. 6D includes long and continuoustrenches 177′ that run in both X and Y directions. Trenches 177′separates islands of pixel electrode layer 170′. FIGS. 6C and 6D are twoexamples. Other configurations and top views of pixel electrode layer170′ are also possible. For example, the top view of top portion oflayer 170′ (islands) shows rectangular-shaped layer 170′ in FIG. 6D.However, pixel electrode layer 170′ of FIG. 6D could be shapeddifferently, such as circular, oval, or other applicable shapes.Similarly, the top view of top portion of pixel electrode layer 170′ ofFIG. 6C could be shaped differently. The islands of pixel electrodelayer 170′ do not need to be rectangular bars. The top view of the topportion of pixel layer 170′ could be curvy bars, or in other shapes.Similar description of configurations can be made for n-type layer 172of FIGS. 4B and 4C.

The existence of the trenches 177′ increases the surface area of N-typelayer 172′ to form PN junction with P-type layer 173′. Similar tophotodiode 135, photodiode 135′ also has improved QE, over flatphotodiodes. In some embodiments, the QE is improved in a range fromabout 20% to about 200%, as compared to photodiodes without trenches (orwith flat surface), which has lower PN-junction interfacial area.

In addition to forming trenches in n-type layer 172 and pixel electrodelayer 170′, as described above, trenches may also be formed in theelectron transport layer 171. The remaining processes involved informing the photodiodes are similar to those described for photodiodes135 and 135′.

In the embodiments described above, the organic photodiodes 135, 135′are used for image sensors. However, the organic photodiodes may also beused for other applications involving photodetectors.

Embodiments of forming an image sensor with organic photodiodes areprovided. Trenches are formed in the organic photodiodes to increase thePN junction interfacial area, which improves the quantum efficiency (QE)of the photodiodes. The organic P-type material is applied in liquidform to fill the trenches. A mixture of P-type materials with differentwork function values and thickness are able to be used to meet thedesired work function value for the photodiodes.

In accordance with some embodiments, a photodiode is provided. Thephotodiode includes a first electrode layer, and an N-type layer. Aplurality of trenches is formed in the N-type layer. The photodiode alsoincludes an organic P-type layer formed over the N-type layer, and theorganic P-type layer fills the trenches in the N-type layer. Thephotodiode further includes a second electrode layer.

In accordance with some other embodiments, a photodiode is provided. Thephotodiode includes a first electrode layer, and an N-type layer. TheN-type layer is formed over a first plurality of trenches. Thephotodiode also includes an organic P-type layer formed over the N-typelayer, and the organic P-type layer fills the trenches in the N-typelayer. The photodiode further includes a second electrode layer.

In accordance with yet some other embodiments, a front-side image sensoris provided. The front-side image sensor includes an organic photodiodewith trenches at the interface between an N-type layer and an organicP-type layer. The front-side image sensor also includes a substrate withdevices and interconnect structures, a color filter layer, and amicro-lens.

In accordance with an embodiment method of forming a photodiode, themethod includes depositing an N-type layer over a first electrode layer.The N-type layer defines a plurality of trenches over the N-type layerincluding: first trenches having respective first lengths disposed in afirst direction in a top-down view of the photodiode and second trencheshaving respective second lengths disposed in a second directionsubstantially perpendicular to the first direction in the top-down viewof the photodiode. The method further includes depositing a P-type layerover the N-type layer in the plurality of trenches and depositing asecond electrode layer over the P-type layer.

In accordance with an embodiment, a method includes patterning aplurality of trenches in a first electrode layer. The method alsoincludes depositing a first doped layer extending along sidewalls andover bottom surfaces of the plurality of trenches. The method alsoincludes depositing a second doped layer over the first doped layer andextending partially into the plurality of trenches. The first dopedlayer includes dopants of a different type than the second doped layer.The method also includes depositing a second electrode layer over thesecond doped layer.

In accordance with an embodiment, a method includes etching a pluralityof trenches in a first electrode layer, depositing an electron transportlayer over the first electrode layer and extending into the plurality oftrenches, and depositing an N-type layer over the electron transportlayer and extending into the plurality of trenches. The method alsoincludes depositing a P-type layer over the N-type layer. An interfacebetween the P-type layer and the N-type layer is disposed in multipleplanes. The method further includes depositing a hole transport layerover the P-type layer and depositing a second electrode layer over thehole transport layer.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A photodiode, comprising: a first electrode; anN-type layer over the first electrode, wherein the N-type layercomprises: a first N-type region; and a second N-type region, wherein aplanar surface of the first electrode extends continuously from directlyunder a first sidewall of the first N-type region to directly under asecond sidewall of the second N-type region, and wherein the planarsurface of the first electrode is opposite the N-type layer; a P-typelayer over the N-type layer, wherein the P-type layer is disposedbetween the first N-type region and the second N-type region, wherein aplanar surface of the P-type layer extends continuously from directlyover the first sidewall of the first N-type region to directly over thesecond sidewall of the second N-type region, and wherein the planarsurface of the P-type layer is opposite the N-type layer; and a secondelectrode over the P-type layer.
 2. The photodiode of claim 1, whereinthe N-type layer further comprises a third N-type region, the P-typelayer is disposed between the first N-type region and the third N-typeregion, a first line parallel to the planar surface of the firstelectrode extends through the first N-type region and the second N-typeregion, a second line parallel to the planar surface of the firstelectrode extends through the first N-type region and the third N-typeregion, and the first line is perpendicular to the second line.
 3. Thephotodiode of claim 2, wherein the planar surface of the first electrodeextends continuously under the third N-type region.
 4. The photodiode ofclaim 1 further comprising an electron transport layer between theN-type layer and the first electrode.
 5. The photodiode of claim 4,wherein the electron transport layer is disposed along sidewalls of thefirst electrode.
 6. The photodiode of claim 1, wherein the P-type layercomprises: a first surface forming an interface with the N-type layer;and a second surface opposite the first surface, wherein the firstsurface of the P-type layer is continuously planar over the first N-typeregion and the second N-type region.
 7. The photodiode of claim 1further comprising a hole-transport layer between the P-type layer andthe second electrode.
 8. The photodiode of claim 1, wherein each layerof the photodiode over the P-type layer is transparent.
 9. Thephotodiode of claim 1, wherein P-type layer comprises an organicmaterial.
 10. A photodiode, comprising: a first electrode comprising: afirst region; a second region physically separated from the firstregion; and a third region connecting the first region to the secondregion; an N-type layer over the first electrode, wherein the N-typelayer extends along a sidewall of the first electrode and along asidewall of the second region of the first electrode; a P-type layerover the N-type layer, wherein the P-type layer extends along thesidewall of the first region of the first electrode and along thesidewall of the second region of the first electrode, and wherein acontinuously planar surface of the P-type layer extends from directlyabove the first region of the first electrode to directly above thesecond region of the first electrode; and a second electrode over theP-type layer.
 11. The photodiode of claim 10 further comprising a firstconductive layer between the N-type layer and the first electrode,wherein a highest occupied molecular orbital (HOMO) of the firstconductive layer is in a range of 6.1 eV to 7.8 eV, and wherein a lowestoccupied molecular orbital (LUMO) of the first conductive layer is in arange of 2.8 eV to 4.5 eV.
 12. The photodiode of claim 11, wherein thefirst conductive layer comprises lithium fluoride, titanium oxide, zincoxide, tantalum oxide, or zinc oxide.
 13. The photodiode of claim 10further comprising a second conductive layer between the P-type layerand the second electrode, wherein a highest occupied molecular orbital(HOMO) of the second conductive layer is in a range of 4.8 eV to 5.6 eV,and wherein a lowest occupied molecular orbital (LUMO) of the secondconductive layer is in a range of 1.5 eV to 3.0 eV.
 14. The photodiodeof claim 10, wherein a highest occupied molecular orbital (HOMO) of theN-type layer is in a range of 6.7 eV to 7.8 eV, and wherein a lowestoccupied molecular orbital (LUMO) of the N-type layer is in a range of3.7 eV to 4.5 eV.
 15. The photodiode of claim 10, wherein a highestoccupied molecular orbital (HOMO) of the P-type layer is in a range of4.5 eV to 5.6 eV, and wherein a lowest occupied molecular orbital (LUMO)of the P-type layer is in a range of 2.8 eV to 3.6 eV.
 16. An imagesensor comprising: an photodiode comprising: a first electrode; anN-type layer over the first electrode, wherein the N-type layercomprises: a first surface forming an interface with a P-type layer, thefirst surface being disposed at multiple levels; and a second surfaceopposite the first surface, the second surface being disposed atmultiple levels; the P-type layer over the N-type layer, wherein asurface of the P-type layer opposite the N-type layer is continuouslyplanar and extends continuously over an entirety of the N-type layer;and a second electrode over the P-type layer; and an interconnectstructure electrically connecting the photodiode to an active device.17. The image sensor of claim 16 further comprising: a color filterlayer on an opposing side of the photodiode as the interconnectstructure; and a micro-lens on an opposing side of the color filterlayer as the photodiode.
 18. The image sensor of claim 16, whereinsurface of the first electrode opposite the N-type layer is continuouslyplanar and extends continuously under an entirety of the N-type layer.19. The image sensor of claim 16, wherein the photodiode furthercomprises an electron transport layer between the first electrode andthe N-type layer.
 20. The image sensor of claim 16, wherein thephotodiode further comprises a hole transport layer between the secondelectrode and the P-type layer.